Seismic surveys image or map the subsurface of the earth by imparting acoustic energy into the ground and recording the reflected energy or “echoes” that return from the rock layers below. The source of the acoustic energy can be generated by explosions, air guns vibrators, and the like. The energy source is positioned on or near the surface of the earth. Each time the energy source is activated it generates a seismic signal that travels into the earth, is partially reflected, and, upon its return, may be detected at many locations on the surface as a function of travel time. The sensors used to detect the returning seismic energy include geophones, accelerometers, and hydrophones. The returning seismic energy is recorded as a continuous signal representing displacement, velocity, acceleration, or other recorded variation as a function of time. Multiple combinations of energy source and sensor can be subsequently combined to create a near continuous image of the subsurface that lies beneath the survey area. One or more sets of seismic signals may be assembled in the final seismic survey.
Technology continues to increase resolution and complexity of seismic systems such as high fidelity vibroseis seismic acquisition including ZENSEIS™. Vibroseis is a method used to propagate energy signals into the earth over an extended period of time as opposed to the near instantaneous energy provided by impulsive sources. The data recorded in this way must be correlated or inverted to convert the extended source signal into an impulse. The source signal using this method was originally generated by a servo-controlled hydraulic vibrator or shaker unit mounted on a mobile base unit, but electro-mechanical versions have also been developed.
Bassett, U.S. Pat. No. 3,972,019, describes accurate timing at distant points where each unit produces synchronized time pulses at time intervals which are the same in the different units and methods for synchronizing the units by direct connection with each other.
Siems and Porter, U.S. Pat. No. 4,281,403, describe a decentralized seismic data recording system including a central station and a plurality of remote seismic recording units. A master clock is provided in the central station. A local clock is provided in each remote recording unit. At the beginning of a work period, the local clocks are synchronized with the master clock. Thereafter, a plurality of seismic data recordings is made. At the end of a work period, the time difference due to tuning drift between the master clock and each respective local clock is ascertained and is recorded. The time difference is linearly prorated over the recordings made during the work period, thereby synchronizing the time base of each seismic data recording with the master clock.
Kostelnicek and Montes, U.S. Pat. No. 4,879,696, describe a method for initiating seismic data storage in an Isolated Distributed Recording System, including the steps of: (a) generating an encoded seismic signal, utilizing an acoustic energy source, which will penetrate to a sub-surface refractive horizon; (b) detecting a refraction of said controlled seismic signal from the refractive horizon with at least one isolated distributed recorder; (c) correlating said refracted seismic signal with a pre-selected correlating signal; and (d) upon correlation between said correlating signal and said refracted seismic signal, triggering the isolated distributed recorder to store incoming seismic data. A Time Synchronization System may be employed for accurately referencing the time of arrival at the isolated distributed recorder of the encoded signal, or all seismic data and information associated with the generation of the encoded signal, to the time said encoded signal or seismic data and information were generated by the acoustic energy source. Generally, such a system would include an accurate clock or other similar timing device at the central station, and each isolated distributed recorder would also have an equally accurate timing device or “local” clock in relationship with the timing device at the central station. Usually, the local clocks are synchronized with the clock at the central station prior to the initiation of active seismic exploration activities.
Reed, et al., U.S. Pat. No. 4,885,724, describe a cableless seismic digital recording system which records seismic-trace data generated by any type of seismic source, including high energy impulsive seismic sources and low energy surface seismic sources such as vibrators. A seismometer is connected to a remotely deployed radio-controlled portable recorder which contains circuitry for sampling, digitizing, processing, storing, and recording seismic-trace data. Coded radio signals instruct each recorder to commence an operation or sequence of operations from a predetermined set of programmed instructions stored in program read only memory included in each recorder. Such operations include seismic-trace data acquisition; optional weighting and vertical stacking; normalization; recording; and seismic source initiation (abstract and p. 8, 1.25-33).
Sallas, et al., U.S. Pat. No. 5,721,710, describe a method where multiple vibratory sources are activated simultaneously and the earth response, for each vibrator-geophone path is obtained. This is done by measuring the vibrations at the sources and at the geophone stations over a number of frequency sweeps. The inverse matrix of the source vibrations is applied to the recorded geophone vibrations at each frequency to derive the transfer function corresponding to the earth response for each vibrator-geophone path in the survey. An earth reflectivity function may be derived for each vibrator-geophone path by applying a minimum phase filter to the separated vibrator-geophone transfer functions.
Harmon, U.S. Pat. No. 6,002,640, describes use of a Series of nearly Identical Seismic Shots (SISS) to generate a system synchronization signal and instruct remote units. The timing and information contained in the SISS can be used to synchronize and communicate with the data acquisition unit(s).
Moldoveanu, U.S. Pat. No. 6,754,590, describes deploying at least one seismic sensor; deploying a plurality of vibratory seismic sources at different source points; simultaneously actuating said seismic sources; acquiring seismic data attributable to said seismic sources using said seismic sensor; redeploying said seismic sources so that at least one of them is positioned at a source point previously occupied by another of them; simultaneously actuating said redeployed seismic sources; acquiring seismic data attributable to said redeployed seismic sources using said seismic sensor; decomposing said acquired seismic data into components attributable to each said seismic source; and stacking together components attributable to seismic sources located at a common source point.
Maxwell, et al., U.S. Pat. No. 6,883,638, describes a method of acquiring seismic data and for operating and testing a sensor assembly. The sensor assembly preferably includes accelerometers with axes of sensitivity orthogonal to each other. The method preferably includes determining sensor tilt angle, determining the position of the sensor, and synchronizing the operation of the sensor (abstract and claim 1). According to one aspect of the patent, an apparatus for synchronizing the operation of a sensor to a common time base is provided that includes a sensor module adapted to sensing seismic energy and the sensor module further includes a global positioning system receiver adapted to synchronize the operation of the sensors. It also includes a seismic recorder coupled to the accelerometers which also includes a GPS receiver adapted to synchronize the sensor.
Iseli, et al., U.S. Pat. No. 7,012,853, describes a method of seismic data acquisition, comprising: a) sensing acoustic energy with a plurality of sensors, each sensor providing an output indicative of the sensed energy; b) collecting a plurality of time samples of each sensor output; c) forming one or more data packets with the collected plurality of time samples: d) adding one or more characterizing bits to the data packets, the characterizing bits representing the time of only the first time sample within the data packet; e) storing the data packets in predetermined memory locations in a field unit; and f) transmitting the data packets. A synchronizing signal is included in the data packets (claims 13 and 16).
Allen, WO9718491 and EP0861450, describe pre-processing seismic data generated by multiple vibrating sources using high resolution or high fidelity data processing. A signal directly related to the actual signal that the vibrator is sending into the ground is used in pre-processing to couple the baseplate motion with signals actually transmitted through the ground.
Longaker, WO0116622, describes a method and apparatus for controlling vibroseis sources in survey operations. A wireless local area network establishing a communications link among vibroseis sources operating in a group may enable the group to operate independent of a remote control unit and may also provide a distributed system solution that mitigates communication difficulties between the sources and the remote control unit (abstract).
Krohn and Johnson, WO2005019865, describe a method for improving the efficiency of acquiring vibratory data with HFVS techniques. With the HFVS method, data from a number of vibrators shaking simultaneously in seismic proximity to one another are separated by using (in one embodiment) a number of phase encoded sweeps, where the number of sweeps is greater than or equal to the number of vibrators, resulting in a set of linear equations that can be solved simultaneously. The record length for each sweep includes an associated listen time containing reflections. The present invention eliminates the unproductive listening time for multiple sweeps but still provides the ability to separate the vibrator records and reduce contamination from harmonics. Production rates can be increased by as much as 30-80% (par.0025).
Willen and Summerfield, WO2007040743, describe controlled source electromagnetic surveys of subterranean regions using two or more electromagnetic transmitters such that the combined responses at a receiver can be separated as determined by waveform periodicities. The methods used to synchronize clocks associated with multiple receivers and transmitters are the same as those methods used to acquire and process data from a single transmitter.
Current systems require contact, either radio or direct contact before, during, and after the seismic survey to synchronize the vibrational sources within microseconds to achieve a uniform signal. Much time and effort is exhausted to synchronize sources across a variety of terrains, under a variety of conditions that are not always amenable to communications either visually or through radio, laser, satellite, cell phone, or other communication methods.
Current systems do not provide an inexpensive and accurate method to synchronize multiple independent sources required for HFVS. Current systems also cannot be used with the variety of source/receiver data formats available for seismic acquisition. What is required are inexpensive and simple methods to synchronize receiver signals from a variety of sources.